Adsorption Of Methylsiloxane By Modified Silica Gel And Biochar Composites

In recent years,the exhaustion of fossil,oil and other natural resources has resulted in energy supply difficulties.The advent of biogas,a renewable and clean source of energy,has led to the development of biogas power generation,heating and other resource utilization technologies.Volatile methyl siloxanes?VMS?are trace impurities of biogas?CH₄,CO₂,and trace amounts of N₂,and CO?produced by the anaerobic digestion of organic waste in landfill and sewage sludge.During the biogas combustion process,VMS are transformed into micro crystalline SiO₂,which can be deposited on the surface of power generation equipment such as,turbine parts,resulting in inefficient heat transfer,excessive component wear and reduced service life.Therefore,the efficient removal of VMS from biogas is a prerequisite for its utilization as an alternative energy resource.To date,the most studied removal process for VMS in biogas was adsorption using solid adsorbents,such as activated carbon,silica gel,zeolite,etc.However,activated carbon,silica gel and other adsorbents have low adsorption capacities,they are difficult to regenerate and promote polymerization of VMS on the surface.Therefore,the main aim of this study was to improve the adsorption capacity of solid adsorbents towards VMS using a series of modified silica gels and coconut shell activated carbon.A study of the relationship between the structural characteristics of the two types of adsorbent and their adsorption capacity may help to elucidate the mechanism of adsorption.Research and development into biological gas purification technology will also provide a theoretical and experimental basis to promote effective biogas energy utilization.Specific areas of study were as follows:?1?Adsorption of hexamethyldisiloxane?L2?by methylated silica gelIn this study,methyl functionalized silica gel?MFS?was prepared by hydrolytic condensation of two precursors,tetraethyl silicate?TEOS?and methyltriethoxysilane?MTS?using methanol-water as the medium.The adsorption of VMS by MFS was then followed using L2 as the model compound.MFS was characterized by cryogenic-nitrogen adsorption using the Brunauer–Emmett–Teller method?N₂-BET?,Fourier transform infrared spectroscopy?FTIR?,cold field emission scanning electron microscopy?SEM?/energy dispersive X-ray spectroscopy?EDS?,X-ray diffraction?XRD?and contact angle.The results showed that MFS had good micropore structure and superhydrophobicity.Among these,the sample prepared with a MTS/TEOS ratio of 1/1?v/v;MFS2?had the optimum properties:the specific surface area was 1261.3 m²·g^-1;the micropore volume was 0.52 cm³·g^-1;the total pore volume was 1.03 cm³·g^-1;the water contact angle was 123.5°;XRD and SEM analysis showed that MFS was amorphous and spongy.The adsorption experiments revealed that the penetration adsorption capacity?Q_B?of MFS was 14.9-18.3 times higher than that of the unmodified silica gel.Q_B of MFS had a linear relationship with specific surface area and pore volume and was positively correlated with total pore volume and water contact angle.An empirical equation describing the change of penetration time?t_B?with the inlet concentration(C_in)was established for the MFS2/L2 adsorption system.The study also showed that the adsorption capacity of the siloxane could be increased by lowering the temperature;at 0°C and C_inn of 83.82 mg·g^-11 L2,Q_B reached 346.3 mg·g^-1.After adsorption of L2 by MFS,it could be regenerated by heating at 80°C;after five successive adsorption/desorption cycles the penetration time remained unchanged.?2?Preparation of micro/mesoporous silica gel modified by polyethyleneimine?PEI?and its adsorption of L2Porous silica gel material?MMSG?was prepared by the sol-gel method using PEI and a silicate mixture as the precursors.The effects of preparation conditions?PEI dosage,solution pH?and adsorption conditions?inlet concentration,adsorption temperature,moisture?on the removal of L2 from MMSG were systematically studied using N₂-BET,SEM,XRD,FTIR and contact angle measurements.The results showed that MMSG prepared at pH=3 and a PEI dosage of 1.0 g gave the optimum conditions for removing L2(t_B=56.3 min,Q_B=289.0mg·g^-1).It was found that t_B and Q_B could reach higher values at low temperature and a low L2 concentration.The Freundlich adsorption isotherm was used to describe the MMSG adsorption process.The coefficient,n=2.45?>1?,indicated that the adsorption process was favored.Using Langmuir adsorption isothermal model to process the adsorption data,it was found that the adsorption process was a homogeneous single layer.The study also showed that the trace water in biogas had no obvious effect on MMSG adsorption of L2.MMSG demonstrated facile thermal regeneration at 100°C;and t_B was maintained between 54.2-56.3 min while Q_B was between 280.3-289.0 mg·g^-1.?3?Effect of organic alcohol modification of inorganic silica gel on the adsorption of L2Silica gel precursors,prepared from Na₂SiO₃ solution,hydrochloric acid and organic alcohol,were aged and roasted to yield a series of organic alcohol modified silica gel materials.The results showed that glycerol modified silica gel,n-propanol modified silica gel and n-hexanol modified silica gel?HA@SSG?adsorbed L2 at t_B of 14.3 min,23.2 min and27.2 min,respectively,indicating that modification with n-hexanol could improve the adsorption performance of L2 to silica gel.When the amount of n-hexanol was 4.0 mL?HA@SSG?b??,optimum adsorption of L2 was obtained;Q_B and maximum adsorption?Q_m?of HA@SSG?b?were 447.6-and 520.2 mg·g^-11 respectively,both of which were twice that of unmodified silica gel.HA@SSG?b?had better adsorption capacity for L2 at low temperature and low L2 concentration.The study also showed that HA@SSG?b?was easily regenerated at80°C and after five adsorption/desorption cycles t_B was stable at 32-36 min,indicating no significant change in adsorption capacity.?4?Adsorption of L”on Fe₃O₄ activated coconut shell carbonHigh specific surface area/high pore volume composites?BC/Fe₃O₄?were obtained by loading Fe₃O₄ on coconut shell biochar?BC?and regulating its texture properties;the adsorption of VMS by BC/Fe₃O₄ was studied using L2.The influence of Fe₃O₄ load,adsorption temperature and inlet concentration on the removal of L2 by BC/Fe₃O₄ was systematically studied using N₂-BET,SEM,XRD and FTIR.The results showed that when the Fe₃O₄ load was about 3 wt%,the composite material?BC-3?attained a maximum specific surface area(1068.43 m²·g^-1)and maximum pore volume(10.36 cm³·g^-1).Adsorption experiments showed that the Q_B of BC-3 was 280.9 mg·g^-11 and Q_m 520.2 mg·g^-1,which were1.71 and 1.78 times higher than that of BC.It was also found that Q_B had a linear relationship with the specific surface area and was positively correlated with the volume of the micropores.At low temperature and low L2 concentration,higher values of t _B and Q_B were obtained.In summary,this study successfully prepared four novel solid adsorbents for the effective removal of VMS:MFS,MMSG,HA@SSG and BC/Fe₃O₄.The adsorption performance parameters?t_B,Q_B and Q_m?of these materials towards L2 were obtained,the adsorption mechanism was clarified,and the recycling performance of the materials demonstrated.This study has evaluated the performance of new VMS adsorbents and provided a theoretical and practical basis for future biogas energy utilization technologies.